It's dust and other hard grit on the brass that eventually wears them out.
Most of my post was said sarcastic.
As far as wearing out dies, I've never been able to accomplish that feat, even after reloading for over 50 years. Can't remember when I got my first carbide tho. Now this is with reloading 20-30 thousand 45 acp cases a summer, and Lord knows how many other calibers.